Crystal growth and scattering studies on two ferrites

Adiga, Shilpa

Book	FZJ-2016-06440

Crystal growth and scattering studies on two ferrites

Adiga, S. (Corresponding author)FZJ*

2016
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-183-5

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich. Reihe Schlüsseltechnologien / Key Technologies 135, IV, 150 S. (2016) = RWTH Aachen, Diss., 2015

Please use a persistent id in citations: http://hdl.handle.net/2128/13020

Abstract: $\textbf{(1) A detailed ferroelectric study of magnetite (Fe$_{3}$O$_{4}$}$) Multiferroics, consisting of both ferroelectric and ferromagnetic phases, have attracted scientic and technological interest due possible magnetoelectric coupling between thephases. Such materials are very rare though, as conventional ferroelectricity requires an empty d-shell, preventing the presence of magnetism. Among unconventional mechanisms leading to ferroelectricity, multiferroicity due to charge ordering (CO) is a strong candidate for practical applications. However, proven examples are very rare as of yet. The 120 K Verwey transition (T$_{V}$) in magnetite, reported in 1939, is the classical example for charge ordering. Despite controversies regarding the existence of CO, magnetite has been proposed as one of the CO-based multiferroics. Although early experiments already indicated for example a magnetoelectric effect, those studies were mainly focused on complex low temperature structure rather than possible multiferroicity. In order to study the ferroelectric properties of magnetite by dielectric spectroscopy, a new dielectric measurement set-up was built at the institute. After an introduction and the description of experimental techniques, this thesis begins with the presentation of our newly built dielectric set-up and of the performed test experiments to standardize measurements of the dielectric constant. The Verwey transition is very sensitive to oxygen stoichiometry. The oxygen stoichiometry was tuned by appropriate gas mixtures of CO$_{2}$ and CO or Ar(H$_{2}$)$_{4}$%. I first investigated appropriate ratios of CO$_{2}$ / Ar(H$_{2}$)$_{4}$% at high temperature on polycrystalline samples and confirmed the phase purity by x-ray diffraction. Verwey transition was characterized primarily by thermo-remanent magnetization and specific heat. The results obtained from the basic macroscopic analysis were used for the growth of high quality crystals by optical floating zone method. Proposed low temperature relaxor ferroelectric property of magnetite was studied by neutron and high energy X-ray diffuse scattering experiments. The observed weak diffuse scattering by neutron diffraction, which was absent in high energy X-ray studies, indicated that it is magnetic in origin. For the first time, a time resolved X-ray diffraction technique has been implemented to test the switchabilty of the polar structure by application of an electric field in magnetite. The observed change in the intensity of the Bragg reflection to its Friedel mate (reflection related by inversion symmetry) constitutes to the first microscopic proof of ferroic behavior of classical magnetite. $\textbf{(2) Study of various physical properties of oxygen deficient strontium ferrite (SrFeO$_{3- \delta}$)}$. Colossal magnetoresistance effect, i.e., the huge change in the electrical resistance by the application of magnetic field is a key to the next generation of magnetic memory devices. The oxygen deficient strontium ferrite (SrFeO$_{3- \delta}$, $\delta$=0-0.5) system exhibits [...]

Note: RWTH Aachen, Diss., 2015

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